Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (=reticle) illuminated via the illumination system is in that projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective to transfer the mask structure onto the light-sensitive coating on the substrate.
To ensure that the light-sensitive resist on the substrate is exposed with the desired amount of light or intensity during the scanning process, it is known, for example in an illumination system, for light to be firstly coupled out at a suitable position in the beam path and for measurement of the intensity distribution then to be performed on that coupled-out light component.
That is diagrammatically illustrated in FIG. 8 which is a diagrammatic view of a structure in principle of a microlithographic projection exposure apparatus 1 having an illumination system 10 and a projection objective 20. The illumination system 10 is used to illuminate a structure-bearing mask (reticle) 30 with light from a light source unit 5 which for example includes an ArF-laser for a working wavelength of 193 nm as well as a beam-shaping optical arrangement producing a parallel light beam. The illumination system 10 includes in a known manner an optical unit 11 having a zoom objective configured to produce a parallel light beam of variable diameter, and an axicon lens. Depending on the respective zoom setting and the position of the axicon lens elements, different illumination configurations are produced by the zoom objective in conjunction with an upstream-disposed diffractive optical element (not shown in FIG. 8), in a pupil plane.
In the illustrated example, the optical unit 11 further includes a deflection mirror 12. The deflection mirror 12 is of a partially translucent nature via a suitable dielectric coating. Light which is coupled out by or which is allowed to pass through the deflection mirror 12 impinges on a dose or intensity sensor 13 for measuring the intensity distribution of that light. Disposed downstream of the optical unit 11 in the light propagation direction, in the beam path, is a light mixing device (not shown) which for example in per se known manner can have an arrangement of microoptical elements which is appropriate to achieve a light mixing effect, as well as a lens group 14. After lens group 14 there is a field plane with a reticle masking system (REMA), the image of which is projected by an REMA objective 15 which is at a downstream position in the light propagation direction onto the structure-bearing mask (reticle) 30 arranged in a further field plane, thereby limiting the illuminated region on the reticle. The structure-bearing mask 30 is imaged with a projection objective 20 onto a wafer or a substrate 40 provided with a light-sensitive layer.
Dose or intensity sensor 13 is used to measure the intensity distribution of the light which is coupled. That measurement operation, as such, does not provide any information about the polarization distribution and in particular it does not provide any information about the levels of intensity of the polarization components in mutually orthogonal directions (for example the x- and y-directions in the illustrated coordinate system).
Among others, US 2007/0146676 A1 discloses coupling the light by a beam splitter arranged in the light path of the illumination system, and analyzing the polarization state of the light.